This invention relates to surgically implantable hip joint prostheses and, more particularly, to a hip joint prosthesis that combines advantages of modular and nonmodular prostheses.
Artificial joints of the human body, including in particular knee and hip joints, have been available for 50 years or more and have been the subject of intense development for at least the last 20 years. The earliest designs provided metal-to-bone or metal-to-metal contact between the articulating surfaces of a joint. Friction and wear were significantly reduced in subsequent designs by the introduction of ultra-high molecular weight polyethylene (UHMWPE) as a load-bearing surface. For example, a typical hip joint prosthesis has an acetabular cup or shell lined with a polyethylene load-bearing surface which is designed to contact a rounded femoral head made of metal.
A number of different methods of securing a polyethylene liner or bearing in an acetabular cup have been developed over the years, as exemplified by the following patents which are incorporated herein by reference in their entireties:
There are two general types of prosthetic components: modular and nonmodular. A nonmodular prosthesis has a bearing secured to a base, e.g., an acetabular cup, during fabrication in the factory, typically by direct compression molding. A modular prosthesis has a prefabricated bearing designed to be attached to the base during surgery.
A modular prosthesis has several advantages over nonmodular prostheses, one of which is that an assortment of different prostheses, i.e., different base/bearing combinations, can be created in the operating room from a small inventory of separate bases and bearings of various sizes, shapes and other characteristics. With a modular prosthesis, an orthopedic surgeon can implant an appropriate base for the patient and then fit the patient with several trial bearings in the process of selecting an appropriate primary bearing to attach to the implanted base. Modular bearings are often readily removable, and in such cases they have the further advantage of facilitating revision surgery, which may become necessary in cases of traumatic injury or bearing surface wear, by enabling replacement of the bearing without removing the base.
There are also advantages to a nonmodular prosthesis construction, such as design simplicity due to the absence of a need for a retaining mechanism for a removable bearing, and relatively low cost. An even more significant advantage is that a nonmodular component is virtually immune to micromotion at the interface between the bearing and the base.
Micromotion is very difficult to avoid with modular components due to the typical need for clearances between bearing and base to ensure that they fit together during assembly in the operating room. A very secure locking method may avoid the problem, but nonmodular fabrication of the prosthesis, e.g., direct compression molding of a bearing onto a base, avoids the issue. Unfortunately, a nonmodular prosthesis has heretofore made revision surgery more difficult in that the entire prosthetic component must be removed and replaced. In addition to the extra operating time involved and extraction tools required. such as described in U.S. Pat. No. 4,459,985 to McKay, removal and replacement of the base requires sacrificing the existing fixation to the bone and has associated complications, including possible bone loss or fracture and the difficulty of reestablishing solid fixation. Nevertheless, it is conventional wisdom regarding a nonmodular prosthesis that the bearing component cannot be changed without changing the base component.
The present invention combines advantages of modular and nonmodular designs by providing a hip joint prosthesis with an integral bearing extraction member for extraction of a bearing from an implanted acetabular cup without disturbing the implanted cup. An extraction member according to the principles of the invention may be implanted as part of an acetabular cup assembly having a modular bearing to facilitate removal of the modular bearing, but is especially useful as part of a nonmodular prosthesis whereby the prosthesis is made convertible in vivo from a nonmodular device to a modular device.
According to one aspect of the invention, the prosthesis includes an acetabular cup having a retainer for a modular bearing, a primary bearing directly molded to the cup, and implantable means for removing the molded primary bearing from the acetabular cup substantially in one piece without disturbing the acetabular cup. A retainer as that term is used herein is a part of the base, i.e., the acetabular cup, that is capable, alone or in conjunction with an auxiliary element or elements, of retaining a modular bearing in place on the base. It may be formed on the base as a one-piece or multi-piece retainer.
According to a further aspect of the invention, a bearing extraction member is mounted in an acetabular cup in contact with a primary bearing as a part of the prosthesis designed to be actuated during revision surgery. Upon actuation, the bearing extraction member forces the primary bearing out of the acetabular cup, after which the primary bearing and extraction member are removed and replaced by a modular bearing. In some cases, conversion to a modular device would be a desirable option even during the primary surgery. It is routine for surgeons to check the patient""s range of motion before and after cementing or otherwise permanently affixing a prosthesis of a chosen size in the patient""s acetabulum. With a nonmodular prosthesis the surgeon has little recourse but to dislodge the complete prosthesis and remove it and any associated cement if the final range-of-motion check is unsatisfactory. The present invention provides a desirable alternative.
A general object of the present invention is to provide an improved surgically implantable joint prosthesis.
Another object of the present invention is to provide advantages of a nonmodular joint prosthesis, including the virtual absence of micromotion, and yet allow a surgeon performing revision surgery to replace a primary bearing without disturbing the base member and thereby jeopardizing fixation.
Another object of the invention is to extend the lifetime of an artificial joint.
These and other objects and advantages of the present invention will be more apparent upon reading the following detailed description of the preferred embodiment in conjunction with the accompanying drawings.